Electrophysiological studies are to be performed on the muscle cells of the proventriculus of the marine polychaete worm Syllis spongiphila. The muscle cells on the dorsal and ventral margins of the proventriculus consist of only a single sarcomere that is about 40 micron in length. The proposed experiments are designed first to characterize the excitable properties of the muscle cells in the intact proventriculus and then to study the development of tension in cells in situ and in isolated single-sarcomere cells. To characterize the excitable properties of the muscle cells, the following topics will be investigated: (a) the degree of electrical coupling between the cells; (b) the identity of the transmitter substances that are released from nerve terminals and activate the muscle cells synaptically; (c) the permeability characteristics of the resting membrane; (d) the ionic fluxes that produce membrane potential changes. To study the development of tension in the muscle cells two main topics will be investigated: (a) the correlation in cells in situ between cell length and membrane potential; (b) the correlation in isolated cells between membrane potential, overlap of thick and thin filaments, tension and various stimulus parameters.